The present invention relates to a vacuum pump and more particularly to a turbo-molecular vacuum pump for relatively high pressure.
Molecular pumps produce a constant pressure ratio in the region of the molecular flow and a constant pressure differential in the region of the laminar flow. In molecular pumps in the style of Gaede, Hollweck or Siegbahn, for example, with very narrow gaps, both the pressure ratio in the molecular region and the pressure differential in the laminar region are particularly high. Turbo-molecular pumps, as a further development of the molecular pumps of earlier design, with larger gaps, produce a very high pressure ratio in the molecular region, but only a small pressure differential in the laminar region.
A molecular pump of Hollweck's design is disclosed, for example, in Swiss Patent No. 222 288. The fundamental construction and the mode of operation of a turbo-molecular pump are described by W. Becker in the journal "Vakuumtechnik", No. 9/10-1966 under the title "The turbo-molecular pump." Both types of pumps are molecular pumps, that is to say, they work in the molecular flow region and the gas transport is effected by transmitting pulses from moved walls to the molecules of the gas to be conveyed.
The working range of turbo-molecular pumps is limited, however, in the direction of higher pressures because they are only fully effective in the molecular flow region. The molecular flow region is limited by the pressure at which the mean free path of the molecules drops to the order of magnitude of the dimensions of the vessel.
Turbo-molecular pumps, therefore, work only in combination with backing or fore pumps. As a rule, these are two-stage sliding-vane rotary pumps. If it were possible to shift the working range of turbo-molecular pumps in the direction of higher pressures, the expense for producing the backing or fore pressure could then be reduced. For example, single-stage sliding-vane rotary pumps would be sufficient. In other cases, oil-sealed sliding-vane rotary pumps could be replaced by dry diaphragm pumps, for example.
The working range of a turbo-molecular pump can be shifted in the direction of higher pressures by fitting a molecular pump of the Hollweck pump type following on the fore vacuum stage. Such combinations are described, for example, in DE-AS 2 409 857 and in EP 01 29 709.
It is essential for the operation of such a Hollweck pump that the spacing between the rotor and stator should be very small. Only then does it work, even at relatively high pressures, as a turbo-molecular pump still in the molecular flow range and develop its full pressure ratio which shifts the working range in the direction of higher pressures. Theory and experimental results call for spacing between the rotor and stator of a few hundredths of millimeters.
Another prerequisite for satisfactory efficiency of a molecular pump is a high speed of rotation of the rotor.
These two extreme requirements, high speed of rotation and narrow gaps, involve two conditions for the design of a molecular pump which are difficult to reconcile with one another. The higher the speed of rotation, the greater must be the minimum spacing between rotating and stationary parts, in order to prevent a collision. With very high speeds of rotation and very narrow gaps, all designs of molecular pumps hitherto known, apart from turbo-molecular pumps, represent extremely critical structural units. This applies in particular when the gap is further reduced by the thermal expansion of the rotor caused by the electric drive, friction losses and compression work. Then the rotor may run against the stator, as a consequence of which destruction of the pump may occur in many cases.